We are examining the effects of psychopharmacologic agents on acquisition of the classically conditioned nictitating membrane response of the rabbit. The nictitating membrane response is a brainstem reflex elicited by delivery of paraorbital shock which, in turn, activates trigeminal inputs to motor neurons innervating the retractor bulbi muscle and, hence, resulting in a retraction of the eyeball and the passive extension of the nictitating membrane over the cornea. The pairing of a conditioned stimulus (e.g., a tone) with delivery of paraorbital shock (the unconditioned stimulus) leads to the acquisition of conditioned responses (extension of the nictitating membrane to presentation of the tone alone). Drug effects are analyzed in terms of whether they are affective associative processes (i.e., learning) or nonassociative processes such as altered baseline rate of responding, sensitization or pseudoconditioning. In addition, drug effects are examined in terms of their possible effects on sensory and motor processes. In order to obtain a more precise specification of the locus of drug effects, we are conducting studies aimed at providing a map of the neural pathways involved in both the conditioned and unconditioned nictitating membrane response. This is being accomplished through the use of anatomical procedures (e.g., retrograde transport of horseradish peroxidase) as well as electrical stimulation and ablation of brainstem nuclei.